Regulation of calcium concentration is the primary mechanism involved in the restoration of the dark state and in the adaptation to background illumination in photoreceptor cells. The primary site of calcium sensing is the protein recoverin. We are studying the structural aspect of calcium induced conformational change in recoverin. For this purpose we have used nitroxide pairs to determine the effect of calcium binding on the relative orientation between the N and C terminal domains and the conformation of the N-terminal helix. Preliminary ESR data indicates a 45-degree rigid body rotation between the two domains upon calcium binding. Furthermore, an increase in the order of the N-terminal helix was detected from analysis of the spectral lineshape of spin labeled single mutants. Our next step will be to measure the kinetics of these structural changes by time-resolved ESR spectroscopy. [unreadable]